Jet propulsion power plant

ABSTRACT

This disclosure pertains to a jet propulsion power plant comprising a gas turbine engine having a first exhaust nozzle whose noise level is kept low by a relatively low pressure/area ratio at the nozzle, and having a plurality of second exhaust nozzles whose noise level is kept low by a pressure/area ratio of the individual nozzles which is sufficiently high to cause the peak of the noise spectrum to be of ultrasonic frequency.

IO n E l2 |3 ,o United States Patent [151 3,677,501 Denning 1 July 18,1972 [54] JET PROPULSION POWER PLANT 3,482,804 12/1969 Pyptiuk ..244/12B 3,240,445 3/1966 Ellzey.... .....244/42 [721 Invent Ralph Deming BnsmhEngland 3,178,887 4/1965 Wilde et al.... .....244/23 D 73 Assignee:RousJhyce Limlted 2,945,641 7/1960 Pribram ..244/1 2 B 3,161,374 12/1964Allred etal ..244/l2 B [22] Filed: March 9, 1970 Primary Examiner-MiltonBuchler [2]] Appl' 17352 Attorney-Mawhinney & Mawhinney [30] ForeignApplication Priority Data [57] ABSTRACT March 8, 1969 Great Britain12,338/69 This disclosure pertains to a jet propulsion power plant eom-March 8, 1969 Great Britain 12,339/69 p sing a gas turbine engine havinga first exhaust nozzle whose noise level is kept low by a relatively lowpressure/area [52] U.S. Cl. ..244/12 B, 244/1 N, 244/55 ratio at thenozzle, n h ing a pl y f second xhaust [51] ..B64c 15/06, B64c 15/14nozzles whose noise level is kept low by a pressure/area ratio [58]Field in Search ..244/12, 23, 42, 53 of the individual mules which issufficiently high to cause the peak of the noise spectrum to be ofultrasonic frequency. [56] References Cited UNITED STATES PATENTS 6Claim, 9 Drawing Figures 3,332,644 7/1967 Whittley .....244/42 cc Y Y 2324 r Patented July 18, 1972 3,677,501

'7 Sheets-Sheet 1 FIG. I.

INVENTOR RALPH MURCH DENNING Patented July 18, 1912 3,677,501

7 Sheets-Sheet 2 FIG. 2.

IO ll Patented July 18, 1972 3,677,501

7 Sheets-Sheet 5 [EIEIEI' E [TE] E21 ii- 3 FIG. 3.

Patented July 18, 1972 7 Sheets-Sheet 4 N DE Patented July 18, 1972 7Sheets-Sheet 5 7 Sheets-Sheet 7 qPNd JET PROPULSION POWER PLANT Thisinvention relates to jet propulsion power plant for aircraft and has forits general object to provide improvements in noise suppression meansfor such plant.

The acoustic frequency of the exhaust jet in a jet propulsion powerplant is a function of the jet velocity and the cross-sectional area ofthe nozzle through which the jet exhausts, the frequency increasing withvelocity and decreasing with area. The higher the acoustic frequency thegreater the attenuation of the noise by the environment and the morenearly will the noise be to the limits of audibility.

Attempts have been made to reduce the noise of a propulsion jet bymodifying the nozzle so as to increase the noise frequency to a pointwhere there is a high rate of attenuation or even inaudibility. This wasdone by causing the jet flow, which normally issues from a singlenozzle, to issue through a plurality of smaller nozzles. However, it hasbeen found that the exhaust of a conventional jet engine, i.e. an enginecomprising in flow series a compressor, a combustor, a turbine and anozzle, does not have sufficient pressure to make it possible to drivethe flow through nozzles of sufficiently small area at sufficiently highvelocities to obtain a useful degree of noise reduction withoutunacceptably high efiiciency losses or size penalties. It is one objectof this invention to reduce or overcome this difficulty.

According to this invention there is provided a jet propulsion powerplant for aircraft, comprising a duct containing in flow-series acompressor, a combustor, a turbine connected to drive the compressor,and a first nozzle, and further comprising a second duct connected tothe first duct to be fed with pressure fluid therefrom, and terminatingin a plurality of second nozzles, characterized in that the second ductis connected to the first duct at a point where pressure is sufficientlyhigh in relation to the individual second nozzles to produce from thesecond nozzles a jet flow the noise spectrum of which has its peak at anultrasonic frequency.

Preferably the second nozzles are arranged in at least one row toprovide access for ambient air to these nozzles and to reduce thefrontal area of the power plant.

The invention is suitable for aircraft capable of vertical or shortaircraft take-off and landing, and in this connection a row ofdownwardly directed second nozzles may be arranged to lie in thedirection of the axis of the compressor and turbine, and in a positiondownstream of the first nozzle and offset from the flow path of the jettherefrom.

It is intended that sound waves of frequencies in the region of 16,000cycles per second should be considered to belong to the ultrasonic rangeof frequencies.

An example of a power plant according to this invention will now bedescribed with reference to the accompanying drawings wherein:

FIG. 1 is a plan view of an aircraft including four such power plants;

FIG. 2 is an enlarged section on the line IIII in FIG. 1;

FIG. 3 is a view in the direction of the arrow III in FIG. 2;

FIG. 4 is a view in the direction of arrow IV in FIG. 2;

FIG. 5 is an enlarged section on the line VV in FIG. 2;

FIG. 6 is a section on the line VI-VI in FIG. 5;

FIG. 7 is a perspective view of a detail of FIGS. 5 and 6;

FIG. 8 is a diagram showing jet noise spectra at different stages ofattenuation; and

FIG. 9 is a diagram relating jet noise generated at different pressureratios.

Referring to FIGS. 1 to 7, the power plant generally denoted P comprisesan engine E including a flow duct 10 containing in flow-series acompressor 11, a combustor 12, a turbine 13 is connected to drive thecompressor, and a primary jet nozzle N1. The nozzle NI is directedrearwards in respect of an aircraft 15 (FIG. I) in which the plant isinstalled and a hood 16 is provided for deflecting the exhaust of thenozzle downwards when required. 1

Only a part of the output of the compressor 11 is led to the combustor12. The remainder of that output is taken off by a flow splitter 20 to aby-pass duct 21 connectable by valves 22 alternatively to a branch duct23 terminating in a rearwardly directed noule 24 and to a branch duct 25terminating in a plurality of downwardly directed secondary nozzles N2.

The nozzles N2 are arranged in two rows (FIG. 3) lying parallel to theaxis of the compressor and turbine and behind and above the nozzle N1.Such an arrangement provides a low frontal area for the power plant andgenerally makes it possible to arrange the large number of nozzles N2 ina manner allowing the ambient air access to the individual nozzles forthe purpose of being induced into the jet flows thereof to contribute tothe noise reduction process.

As shown in FIGS. 5, 6, the flow connection between the duct 25 and thenozzles N2 includes combustion chambers 27 having fuel supply means 28for heating the flow to the nozzles N2. The flow from the combustionchambers 27 is divided by a rectangular center body 31 into rectangularopenings 29 which are subdivided by flow splitters 30, and the actualarea of one nozzle N2 is the area of an opening 29 between two suchsplitters or between the end of the opening and the nearest splitter.

The center bodies are hollow and each define a passage 32 for theinduction ambient air as shown by arrows 33.

The space below the nozzles N2 is shielded by retractable flaps 34against the spread of noise from the rather wide noise front presentedby the arrangement of the nozzles as seen in FIG. 2.

As shown in FIG. 1, the aircraft 15 is provided with four power plants Psymmetrically spaced about the longitudinal axis of the aircraft. Theducts 25 of the individual plants are connected by a duct 35 so that,when the nozzles N2 are used, failure of the engine in one of the plantswill not deprive the nozzles N2 of that plant of a pressure supply.

In operation, for the purpose of vertical take-ofi, the hood 16, andshields 34 are moved by appropriate actuators into the positions shownin full lines in the drawings. For transition to forward flight thevalves 22 are moved to the position shown in full lines to direct theby-pass flow through the branch duct 23.

The engine may be of a single spool or a multi-spool type and thejunction to the duct 21 may be arranged where necessary along thecompressor or compressors to obtain the desired pressure ratio for theduct 21.

The combustion chambers 27 are not essential in cases where a highpressure ratio is available and only a relatively low thrust isrequired, and possibly where a sacrifice is acceptable in thepower/weight ratio of the engine.

It will be noted that the power plant combines horizontal and verticalthrust capability on the basis of a single engine, and includes thenoise reduction facility which is so important for vertical thrustapplications.

The division of the plant in such a way that the gas flow is exhaustedthrough one relatively large primary nozzle N1 and a plurality of smallsecondary nozzles N2 makes it possible to achieve an overall noisereduction not achievable if the whole of the exhaust of the power plantwere passed through the nozzle N1. It will be seen 1) that therelatively large turbine pressure ratio necessary to drive a compressorcapable of feeding both the combustor 12 and the by-pass 21 makes itpossible to depress the jet velocity, and thus the noise, at the nozzleN1 and (2) that the high pressure ratio at which the duct 21 is fedmakes it possible to raise the velocity/area ratio and thus thefrequency of the jet noise at the noules N2 to an extent where asignificant proportion of the noise is either atmospherically attenuatedor becomes inaudible. The calculations necessary to bring about theseconditions are essentially known to those skilled in this art and it isnecessary here only to point out those steps which have a particularbearing on this invention.

The calculations start with a known requirement for total thrust in thevertical direction and initial assumptions about the total mass flowrequired, the by-pass ratio (i.e. the by-pass ratio is dependent on theratio of the mass flows through the main and the by-pass ductrespectively), the pressure and temperature ratios for the main duct,the pressure and temperature ratios for the by-pass duct, and so on.

Separate calculations are then made for the main and the by-pass duct todetermine jet velocities, total thrust, specific thrust, specific fuelconsumption and noise levels. The two sets of calculations are thencompared and if necessary repeated to obtain the best compromise betweenoverall fuel consumption, thrust, weight and so on. This iterativeprocess is well known per se.

In the calculations for the main duct the jet velocity is keptsufficiently low to avoid excessive noise levels and if, in the initialcalculation the pressure chosen for the main duct resulted in too high ajet velocity, then the by-pass ratio is lowered to lower the main ductjet velocity. This consideration determines the position along the mainduct at which the flow is divided, i.e. if a lower pressure ratio isrequired for the main duct, the junction with the by-pass duct isselected to lie further upstream along the compressor, and vice versa. Atypical figure for a jet velocity for the main duct is 950 ft/sec.

In the calculations for the by-pass duct the aim is not low but high jetvelocity and a set of typical figures is now described by way ofexample.

Assuming a by-pass pressure ratio of 15:1 and a nozzle entry temperatureof l,000l(, the calculable jet velocity is about 3,500 ft/sec. At thisvelocity a mass flow giving a thrust of 20,000 lbs. requires acalculation total nozzle area of 68 square inches. Assuming this to bedivided into 346 nozzles of 0.5 inch diameter (about 0.2 square incharea), it is then possible to calculate the frequency at the peak of thenoise spectrum from the simple relationships:-

Jet Velocity StrouhaLN umber The term for Strouhal number isapproximately 0.8 at the angle of peak noise radiation and the frequencyis, in the present example, approximately 16,000 cycles per second whichis above the normal range of human hearing.

FIG. 8 shows at A the uncorrected source noise spectrum for the soundpressure level of the 346 nozzles at 1,500 ft from the point ofobservation, and at B the same spectrum corrected for atmosphericattenuation. C and D show the corresponding spectrum for a single nozzleof 68 square inch area and demonstrate how although the uncorrectedpressure level is the same in both cases (i.e. at A and C), the peaklevel of the attenuated noise for the 346 nozzles is significantly lowerthan in the case of the single nozzle.

FIG. 9 shows a set of curves showing the effects of changing pressureratios on the noise produced by the 346 nozzles, together with theeffects of distance from source of noise. It will be noted how the noiselevel rises with reduction in pressure ratio. In other words, unless arelatively high pressure ratio is available, a very large number of verysmall nozzles would have to be introduced to attain significant noisesuppression. It has been found that at pressure ratios below :1 andnozzle areas below 0.2 square inch it is not practicable to seek animprovement of the noise problem by raising the noise frequency. This ispartly because the nozzle efficiency itself falls with the increasingpreponderance of boundary layer and partly because of problems ofpractical construction involving a large number of small outlets. Evennozzle areas of the order of 0.2 square inch require special structuralorganization such as shown in the drawings to make them practicableespecially as regards the induction of ambient air.

Just as there are practical limits to the reduction in nozzle area, sothere are practical limits to increasing pressure ratio, temperature andjet velocity. The figures mentioned (15:1 for pressure ratio, l,000l(for temperature and 3,500 ft/sec for jet velocity) are well within thecapacity of present-day technology, but with improvements in componentdesign and in refractory materials higher values can be contemplated.

What we claim is:

1. Jet propulsion power plant for aircraft, comprising a duct containingin flow-series a compressor, a combustor, a turbine connected to drivethe compressor, and a first primary jet nozzle, and further comprising asecond duct connected to the first duct to be fed with pressure fluidtherefrom, and terminating in a plurality of secondary jet nozzles, saidsecond duct being connected to the first duct at a point in the vicinityof the compressor where pressure is sufficiently high in relation to theindividual secondary nozzles to produce from the secondary nozzlesindividual and distinct jet flows having a noise spectrum which has itspeak at an ultrasonic frequency, said secondary nozzles including aplurality of combustion chambers having an outlet for combustionproducts and comprising a center body of substantially rectangular form,as seen in the general direction of flow through the outlet, anddischarge passages for the combustion products arranged at the longsides of the said form, the center body being hollow and having interiorsurfaces defining a space open to ambient air at, at least, one of theshort sides of said form and at the downstream end of the center body.

2. Power plant according to claim 1 wherein the secondary nozzles arearranged in at least one row.

3. Power plant according to claim 2 wherein the secondary nozzles arearranged to be directed downwardly (in respect of an aircraft), to liein a row extending in the direction of the axis of said compressor andturbine, and to be situated downstream of the first nozzle off-set fromthe flow path of the jet therefrom.

4. Power plant according to claim 3 comprising means for deflecting thejet flow of the first nozzle from a rearward direction (in respect of anaircraft) into a direction having a downward component.

5. Power plant according to claim 2 comprising a retractable shieldarranged parallel to the length of the row and downstream of thesecondary nozzles in a position to inhibit the spread of jet noiselaterally in respect of the row.

6. Power plant according to claim 1 comprising a third duct connected tothe secondary duct at a point between the first duct and the secondnozzles, a third nozzle connected to the third duct and valve means fordiverting the flow in the second duct to by-pass the secondary nozzlesand to be discharged through the third noule.

lOlO45 0106

1. Jet propulsion power plant for aircraft, comprising a duct containingin flow-series a compressor, a combustor, a turbine connected to drivethe compressor, and a first primary jet nozzle, and further comprising asecond duct connected to the first duct to be fed with pressure fluidtherefrom, and terminating in a plurality of secondary jet nozzles, saidsecond duct being connected to the first duct at a point in the vicinityof the compressor where pressure is sufficiently high in relation to theindividual secondary nozzles to produce from the secondary nozzlesindividual and distinct jet flows having a noise spectrum which has itspeak at an ultrasonic frequency, said secondary nozzles including aplurality of combustion chambers having an outlet for combustionproducts and comprising a center body of substantially rectangular form,as seen in the general direction of flow through the outlet, anddischarge passages for the combustion products arranged at the longsides of the said form, the center body being hollow and having interiorsurfaces defining a space open to ambient air at, at least, one of theshort sides of said form and at the downstream end of the center body.2. Power plant according to claim 1 wherein the secondary nozzles arearranged in at least one row.
 3. Power plant according to claim 2wherein the secondary nozzles are arranged to be directed downwardly (inrespect of an aircraft), to lie in a row extending in the direction ofthe axis of said compressor and turbine, and to be situated downstreamof the first nozzle off-set from the flow path of the jet therefrom. 4.Power plant according to claim 3 comprising means for deflecting the jetflow of the first nozzle from a rearward direction (in respect of anaircraft) into a direction having a downward component.
 5. Power plantaccording to claim 2 comprising a retractable shield arranged parallelto the length of the row and downstream of the secondary nozzles in aposition to inhibit the spread of jet noise laterally in respect of therow.
 6. Power plant according to claim 1 comprising a third ductconnected to the secondary duct at a point between the first duct andthe second nozzles, a third nozzle connected to the third duct and valvemeans for diverting the flow in the second duct to by-pass the secondarynozzles and to be discharged through the third nozzle.